1. Field of the Invention
The invention relates to a sealed battery and a method for manufacturing the same.
2. Description of Related Art
One representative structure of a battery is a structure in which a power generating element formed by a positive electrode and a negative electrode stacked together with a separator in between is housed in a battery case, and this battery case is sealed (i.e., a sealed battery). Typically with this sealed battery, the inside of the battery case is sealed by closing off (sealing) an opening of the case main body with a suitable sealing member (for example, a sealing plate) after the power generating element is housed in a case main body. One method for sealing the opening of this battery case involves arranging the sealing member in a position closing off the opening, and then joining the sealing member to an edge of the opening by welding, for example.
The shape of the joint between the edge of the opening of the case main body and the sealing member is typically a corner joint in which an edge portion of one member is butted against an inside wall of the other member. The butted portions of these two members are then welded from the outside of the angle formed by the two members.
When sealing the battery case by joining a sealing member (for example, the sealing plate) that seals the opening of the case main body to the edge of the opening by welding, the weld strength between the case main body and the sealing member (e.g., the sealing plate) may decrease if the weld depth of a weld portion (i.e., a melt-solidified portion that has been melted at the time of welding and then solidified; hereinafter referred to as such) is insufficient at the butt portion of the case main body and the sealing member. Thus, Japanese Patent Application Publication No. 2009-218099 (JP 2009-218099 A) and Japanese Patent Application Publication No. 11-104866 (JP 11-104866 A), for example, describe technology for increasing this weld strength.
JP 11-104866 A describes technology that ensures weld strength of a butt portion of a case main body and a sealing member, by performing welding using a high output, high convergence laser, and increasing the weld depth of the weld portion at the butt portion of the case main body and the sealing member.
However, with the technology described in JP 11-104866 A, if the welding position (typically a laser irradiating position) is off, a desired weld depth may not be able to be ensured at the weld portion of the butt portion of the case main body and the sealing member (i.e., the boundary between the case main body and the sealing member). That is, weld strength at the boundary between the case main body and the sealing member may not be able to be sufficiently ensured due to variation in the battery manufacturing process (e.g., variation in the welding position, variation in the sizes of the case and sealing member, etc.).
Also, if the weld depth of the weld portion is excessive and the location that has melted at the time of welding (i.e., the melted portion) ends up reaching (ends up passing through) the inside wall of the battery case, foreign matter (typically metallic particles) from spatter may end up getting into the battery case at the time of welding. Foreign matter (spatter) that gets into the battery case may cause an internal short in the battery, and is thus undesirable. Also, if the weld depth of the weld portion is too great, a bubble (cavity) is likely to remain after the melted portion solidifies, i.e., a so-called void (also referred to as “blowhole”) is likely to form. This void (cavity) reduces the strength of the weld location (weld strength), which leads to a decrease in weld quality and is therefore undesirable. In recent years, the case main body and the sealing member have become progressively thinner in view of increasing battery capacity and reducing battery weight and the like. From this as well, there are cases in which it is difficult to ensure sufficient weld strength only by increasing the weld depth of the weld portion as described in JP 11-104866 A.
JP 2009-218099 A describes technology that ensures the weld depth of a weld portion at a boundary (butt portion) of a case main body and a sealing member, by giving the sealing member a predetermined shape. With the technology described in JP 2009-218099 A, a boundary portion of a non-weld portion and a weld portion of the case main body is parallel to a thickness direction of the case main body, and the area of the boundary portion of the non-weld portion and the weld portion of the case main body is small. Typically, the mechanical strength of the boundary portion of the non-weld portion and the weld portion tends to be less than the mechanical strength of the weld portion and the mechanical strength of the non-weld portion of a welding portion. Therefore, with the technology described in JP 2009-218099 A, even though the joint strength of the boundary (butt portion) of the case main body and the sealing member is able to be increased, there are cases in which it is difficult to ensure sufficient weld strength at the boundary portion of the non-weld portion and the weld portion of the case main body.